The present invention relates to a superconducting coil refrigerating method and a superconducting apparatus, and more particularly to a superconducting coil refrigerating method and a superconducting apparatus suitable in the case where a pulse magnet run with fast magnetization/ demagnetization or with the repetition of magnetization and demagnetization is used.
Up to this day, there are various articles which explain refrigeration methods in superconducting devices. Technical & Research Report (Section II) No. 93 by the Institute of Electrical Engineers of Japan describes on and after page 61 an immersion refrigeration method using liquid helium and a forced refrigeration method using liquid helium (more especially, forced refrigeration by supercritical helium).
The bath cooling method using liquid helium is the most general refrigeration method. In this method, a superconducting coil is immersed in a helium tank filled with the liquid helium so that the superconducting coil is refrigerated by virtue of an effervescence heat transfer characteristic of the liquid helium. A steady state (including a liquid helium storing state and a conducting state) involves only a natural convection. Therefore, in the immersion refrigeration method, it is required that liquid helium with the amount corresponding to that of liquid helium evaporating from the helium tank due to a thermal penetration of the superconducting coil is supplied as required or continuously.
On the other hand, in the forced cooling method using liquid helium, the liquid helium is forcibly flown into or outside of a superconductor forming a superconducting coil so that the superconducting coil is refrigerated by virtue of a forced convection heat transfer characteristic of the liquid helium. Advanced development of the forced cooling method has been made since the forced convection heat transfer in the forced cooling method has a large refrigeration ability in comparison with the effervescence heat transfer in the bath cooling method. However, the forced cooling method is not yet popular as compared with the bath cooling method. In the case of the forced cooling method, the flow of liquid helium is forcibly formed always over the initial refrigeration stage of the superconducting coil, the liquid helium storing state and the conducting state.
In order to ascertain the stability of a superconducting coil, the refrigeration of the coil is one of important tasks. Especially, in the case of a pulse magnet which is run with fast magnetization/demagnetization or with the repetition of magnetization and demagnetization, the problem of refrigeration is particularly important since there are always present the generation of heat from the coil itself due to an AC loss generated from a superconductor itself and a structure surrounding it and the generation of helium gas bubbles attendant upon the heat generation.
In the light of the above point of view, the above-mentioned conventional refrigeration methods have the following problems to be solved with respect to the superconducting pulse magnet. Namely, in the case of the bath cooling method, though a stable refrigeration characteristic is obtained because the liquid helium is stagnant or not flowing, there is a problem that the transfer (or migration) and exhaustion of helium gas bubbles generated is difficult and hence the stagnation of the bubbles causes the deterioration of effervescence heat transfer characteristic at the surface of the superconductor and hence the degradation of the stability of the superconducting coil. In the case of the forced cooling method, on the other hand, though it is advantageous in the improvement of the refrigeration performance and the migration or movement of helium gas bubbles owing to the forced convection heat transfer, there is a problem of uncertainty attendant upon the flow of liquid helium or a possibility that a change of flow distribution in and/or a flow stagnation in parallel channels take place. As a result, the reliability of the forced cooling method is questionable and hence it is difficult to keep the stability of the coil continually. Further, the continuous forced flow of liquid helium causes a so-called flash loss. Namely, a pressure is imposed on the liquid helium so that the liquid helium is partially gasified, thereby degrading the quality of the liquid helium. Such a flash loss is not preferable for the refrigeration characteristic of the coil.